1. Field of the Invention
The present invention relates to an optical glass suitable for highly precise press molding at a relatively low temperature.
2. Background Art
In recent years, digital cameras and portable telephones with a camera of taking image data by imaging device such as CCD (Charge Coupled Device) and CMOS (Complementary Metal Oxide Semiconductor) are rapidly prevailing. In particular in recent years, imaging device having a large pixel number have been developed for achieving high picture quality and, with such a tendency, high optical performance has been required of imaging lenses. On the other hand, demand for miniaturization is also increasing.
For answering such requirements, glass mold lenses press molded by means of a metallic mold having a highly precise dimension are adopted in many cases as the above imaginglenses. According to such press molding, as compared with processing by polishing, an optical lens having an aspheric surface and an optical lens of a micro-dimension can be easily and efficiently manufactured.
Since such press molding is performed at a temperature higher than the deformation temperature of the optical glass as a raw material, a metallic mold hardly subject to physical load such as heat and stress is required to have high durability. Of course, the higher the deformation temperature of the optical glass, the higher is the physical load to the metallic mold. Accordingly it is necessary to suppress the deformation temperature of the optical glass as low as possible for the purpose of lengthening the duration of the metallic mold.
On the other hand, optical glasses having a high refractive index and low dispersibility are also eagerly required to advance miniaturization and angle-widening of imaginglenses.
From such a background, some optical glasses having a relatively low deformation temperature (and a glass transition temperature) in spite of high refractive index and low dispersibility have been developed (for example, refer to JP-A-2000-16831, 2000-119036, and 2001-130924). JP-A-2000-16831, 2000-119036, and 2001-130924 disclose the optical glasses mainly consist of silicon oxide (SiO2), boron oxide (B2O3), and lanthanum oxide (La2O3).
However, in recent years, miniaturization and improvement in performances of imaging lenses are conspicuously advanced and further increase in refractive index, low dispersibility, and processing easiness of the optical glass are demanded.